An explosion ripped through New Zealand’s largest coal mine on Friday with about 30 people underground, officials said. Five workers, dazed and slightly injured, stumbled to the surface, while more than two dozen were missing.

Police said the electricity in the mine went out shortly before the blast — which was powerful enough to blow one driver off his machine deep in a mine tunnel — and this may have caused ventilation problems. One mine safety expert said a gas explosion was a possible cause.

Police spokeswoman Barbara Dunn stressed it was too early to say why the explosion occurred.

Rescue teams and emergency workers rushed by helicopter and by road to the mine, located in remote and rugged mountains near the town of Atarau on New Zealand’s South Island.

“It’s not good news at all,” Tony Kokshoorn, the mayor of nearby Greymouth, told National Radio. “We don’t know at what depth the explosion is but there’s certainly a big explosion. With a bit of luck, things might be OK.”

Mr. Kokshoorn put the number of miners unaccounted for at up to 30. Mine operator Pike River Coal initially said the number could be around 30, but chief executive Peter Whittall later amended that, saying 27 people were missing — 15 miners employed by the company and 12 local contractors.

Mr. Whittall said five workers had walked out of the mine — a pair that included a machine operator who was blown off his vehicle 5,000 feet (1,500m) underground, and, later, three more. One of the men had been able to make a call on his cell phone before reaching the surface, he said.

Mr. Whittall said he was not aware of any deaths caused by the blast.

Energy Minister Gerry Brownlee said the explosion happened at about 3.45 pm and the last contact with any of the miners was about half an hour later.

The men who came to the surface were taken to a hospital for treatment of light injuries.

“They’re being interviewed and we’re trying to determine... the full nature of the incident,” Mr. Whittall said.

It was not immediately clear if all of those underground were together or in separate groups.

“There is concern that ventilation inside the mine shaft may be compromised by the power outage,” Ms. Dunn said.

Mine safety expert David Feickert said the blast could have been a gas explosion.

“There are different kinds of explosions that can occur in a coal mine — methane gas, coal gas and so on,” he said. “If rescue teams can go in, that’s good news indeed.”

Rescue crews were assembling at the opening of the mine but had not yet entered.

Police area commander John Canning said details were still sparse but initial reports suggested those still in the mine could be up to 5,000 feet (1,500m) underground.

Mr. Brownlee said emergency exit tunnels were built into the mine but that he didn’t know if they could be accessed by the miners.

St. John Ambulance service said three rescue helicopters and six ambulances had headed to the mine.

The coal seam is about 200 meters underground and is reached through a horizontal tunnel 2.3 km in length. One vertical ventilation shaft rises 354 feet (108m) from the tunnel to the surface, according to the company’s Website. This was blocked by falling rocks within the shaft in early 2009, delaying mining for months.

Pike River has been operating since 2008, mining a seam with 58.5 million tons of coal, the largest-known deposit of hard coking coal in New Zealand, according to its website.

Pike River says its coal preparation plant at the site is the largest and most modern in New Zealand and processes up to 1.5 million tonnes of raw coal a year.

The mine is not far from the site of one of New Zealand’s worst mining disasters — an underground explosion in the state-owned Strongman Mine on January 19, 1967, that killed 19 workers.

Mr. Whittall said the horizontal mine tunnel would make the rescue effort easier than if the shaft was at a steep angle.

“We’re not a deep-shafted mine so men and rescue teams can get in and out quite effectively, and they’ll be able to explore the mine quite quickly,” he said. “They will work throughout the night and they’ll work until they can go right throughout the mine and determine the extent of the incident and the safety of our employees.”

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